Scanning antenna

ABSTRACT

8. The combination of a lens system having an optical axis and a field of focus, an antenna for the radiation or reception of energy, a dielectric block between said lens and said antenna having opposed parallel plane surfaces and rotatable about an axis normal to said optical axis and parallel to said surfaces, the index of refraction of said block with respect to the surrounding medium having a value to bring the apparent position of said antenna as a source viewed from said lens system substantially into coincidence with said field, as said block rotates.

arts- 759. (pa-75 United States Patent [191 Wilkinson, Jr.

June 3, 1975 SCANNING ANTENNA [75] Inventor: William C. Wilkinson, Jr.,

Princeton, N.J.

[73] Assignee: RCA Corporation, New York, NY.

[22] Filed: June 30, 1950 [21] Appl. No.: 171,448

[52] U.S. Cl 343/754; 343/786 [51] Int. Cl. H01q 19/06 [58] Field of Search... 250/3363, 33.63 F, 33.63 L,

[56] References Cited UNITED STATES PATENTS 2,078,302 4/1937 Wolff 343/838 2,422,579 6/1947 McClellan 343/754 2,511,610 6/1950 Wheeler 343/783 Primary Examiner-Maynard R. Wilbur Assistant Examiner-Richard E. Berger Attorney, Agent, or F irm-Edward J. Norton [57] EXEMPLARY CLAIM 8. The combination of a lens system having an optical axis and a field of focus, an antenna for the radiation or reception of energy, a dielectric block between said lens and said antenna having opposed parallel plane surfaces and rotatable about an axis normal to said optical axis and parallel to said surfaces, the index of refraction of said block with respect to the surrounding medium having a value to bring the apparent position of said antenna as a source viewed from said lens system substantially into coincidence with said field, as said block rotates.

10 Claims, 3 Drawing Figures SCANNING ANTENNA This invention relates to scanning antennas, and preferably to such antennas for use at microwave wavelengths of less than ten centimeters.

For radar (radio echo detection and ranging apparatus) and similar purposes, it is sometimes required to scan a sector of space with a radio beam. It is usually desired to move the beam at a linear rate across a certain angle. Ordinarily, the beam must be as narrow as possible, particularly in the direction of the scanning action, in order to provide good angular definition. In the copending application of C. H. Chandler, Ser. No. 783,174, filed Oct. 30, 1947, now abandoned, there is disclosed an apparatus for securing the scanning of 'space. The present application is an improvement over the apparatus claimed in the said Chandler application.

It is an object of the invention to improve the methods and apparatus for scanning a sector of space with electromagnetic energy.

It is another object of the invention to improve scanning devices of the type in which the scanning is linear with time in its angular motion.

A further object of the invention is to improve the electrical characteristics of scanning apparatus.

A still further object of the invention is to improve the focusing properties of scanning apparatus.

A further object of the invention is to improve scanning by scanners of the dielectric block or prism types.

These and other objects, advantages, and novel features of the invention will be more apparent from the following description when taken in connection with the accompanying drawing in which:

FIG. 1 is a perspective view of one embodiment of the invention;

FIG. 2 is a schematic drawing of a dielectric scanning block useful in understanding certain principles of the invention; and

FIG. 3 is a graph useful in understanding certain relationships preferred in the practice of the invention.

According to the invention, a scanning system using the principles of a dielectric block scanner is employed. In the preferred form of the invention, the dielectric block is associated with a rotor stack of parallel metallic plates, to increase the equivalent index of refraction of the block. A stack of stator plates suppresses aberrations which might otherwise prove troublesome. Further, the motion of the apparent source is made to conform to the field of focus of a lens system by appropriate choice of the effective index of refraction.

Referring now more particularly to FIG. 1, a horn antenna may be supplied with energy from a source cosa outer circular edges 24 also aligned. The stacked rotor plates 14 are mounted for rotation about a center 26 and positioned between antenna 10 and a focusing device such as a lens 27. A further stack 28 of parallel metallic stator plates are fixedly supported by suitable means (not shown) and have circular concave edges 40 closely adjacent and complementary to the rotor circular convex edges 24. The space 30 between rotor plates 14 and stator plates 28 is so small as to result in a negligible effect in refracting or bending rays of energy. The rotor and stator plates 14 and 28 are equally spaced apart and have the same effective index of refraction with the complementary circular edges 24 and 40 placed with the edges substantially in registry to contribute to this result. Of course, the energy refracted by the stacks of plates is polarized with the electric vector parallel to the plates. A dielectric block 32 is supported in the openings 18, preferably by the dielectric members l6, and is spaced from the metallic plates 14 by a quarter-wave space or margin 35. The block 32 thus has plane surfaces such as 34 spaced from the straight edges such as 20 and parallel thereto. The rotor plates 14 and block 32 together thus rotate about axis 26 parallel to the plane surfaces of the block 32.

It is already known that a dielectric block placed in front of an antenna 10 or other radiator with its center of radiation on the optical axis may be rotated to give an apparent displacement of the source. Consider a hypothetical block 32' illustrated in FIG. 2, of thickness T, through'which radiation from a point source P, a distance R along the axis 38 from the block center 0, is to be observed from the right as viewed in FIG. 2 parallel to the axis 38 by an observer in the medium 43. Refraction creates an apparent source P". Assume the block to be rotated an angle a about its center 0 to another position, shown in dotted lines. The apparent source P then moves to P at a distance CI from the axis 41 through 0 normal to the optical axis 38. The distance Cl is given by the following equation:

Cos 2a (n sin m sin 2a cosa where n is an effective index of refraction between the block 35 and the surrounding medium 43 at the operating frequency.

Referred to the particular point P" at which P intersects the axis 38 in its motion, the axial displacement C is given by:

by simple translation of the origin.

It is now possible to secure a rough measure of the index of refraction between the hypothetical block 32' and the space about it at which the displacement C of the point P from a straight line is not very great. This may be done, for example, by choosing some maximum angle of departure of the parallel prism faces 34 from the position of normal with respect to the optical axis, at which the block face 34 is to refract the energy, such at 45. The effective index of refraction n is then determined for which the point P (when the block is dis cos 2a -sin er) placed through the angle of 45) is at the same displacement axially as when it crosses the center axis 38. That is, with a at 45, (or an appropriate maximum angle), n is chosen to make C zero. FIG. 3 illustrates a curve, with 01 at 45, of the axial displacements, C plotted against the index of refraction n. The required effective index n is in this case about 3.3, for C to be zero at a 45 as well as at 0. It is apparent that the realization of such large effective indices of refraction in usable arrangements is difficult of attainment. Of course, it may happen that a smaller index will suffice, and it then is unnecessary to have the complex structure to achieve bringing the antenna as a virtual or apparent source as viewed from the lens substantially into coincidence with the field of the lens, as the block rotates.

Referring, however, to FIG. 1, the relative or effective index between the block 32 and the stack of plates 14 may be readily made of the desired order of magnitude by following the invention. Thus, the stack of plates may have an effective index of refraction of the order of 0.625 with air, while the dielectric block 32 or prism as it is sometimes called, may have an index of refraction of 1.6 with respect to air. Thus, the dielectric block 32 has an index of refraction of about 2.56 with respect to the surrounding medium, the stack of metallic plates. The quarter-wave matching space of air has a dielectric constant of unity equal to the geometric mean V1.6 X .625) between these. Thus, optimum results and minimum reflection at the interfaces of the matching section are achieved. The stack 28 of stator plates on the side facing lens 27 have straight edges 42 normal to the axis of the device and normal to the general direction of energy. Without the circular edges 40 and the straight edges 42 of the stator plates 28, which have the same spacing and effective index of refraction as the rotor stack of plates 14, the device would be subject to serious aberrations. Therefore, the stack of stator plates 28 affords a substantial improvement which avoids these aberrations, because emanation of energy from the straight line 42 into the air does not seriously affect the rays from the apparent source.

Furthermore, the straight stator edges 42 and the arrangement of FIG. 1 does not adversely affect or seriously change the analysis illustrated in FIGS. 2 and 3. Its principal effect is to displace the apparent source axially parallel to axis 38 or the corresponding optical axis of FIG. 1. The corresponding change in FIG. 2 may therefore be expressed simply as a correction of Cl. For example, the equivalent hypothetical structure in FIG. 2 would have the prism 32' immersed in a medium 43 having as viewed in FIG. 2 a plane interface 45 with a medium, say air 47. Instead of Cl, then, an observer in medium 47 should use CI+D( ln,) T

for the distance of an apparent source (not shown) from axis 41. In equation (3), n is the effective index of refraction of the medium 43 referred to that of medium 47 corresponding in FIG. 1 to the effective index of the stack of plates 28 referred to the medium beyond (to the right as viewed in FIG. 1) the edges 42. This index n is then practically always referred to air. D is the distance from the center 0 in FIG. 2 (26 of FIG. 1)

of rotation to the interface 45 in FIG. 2 (edges 42 of FIG. 1). The net effect of the straight edges 42 then is to require a shifting of the focusing device or lens 27 axially along the optical axis with respect to the remainder of the apparatus to cause the apparent source to coincide as closely as possible with the focal surface of lens 27.

In discussing the device, it has been assumed that a fiat field is desired, or a substantially fiat field. It will be understood that a preferred condition may be one in which the curvature of the field or the movement of the apparent source P substantially conforms to the curvature of the field of the lens 27. One way of assuring this result is by ray tracing or more accurate analysis. The larger effective index of the prism contributes to the easy attainment of flatter fields.

It is apparent from the foregoing, that there has been disclosed a novel scanning device which overcomes to a considerable degree the difficult problem of aberrations to which the prior art scanning devices have been subject. The combination of the dielectric block with the stacked plates permits the achievement of a sufficiently large effective index of refraction between the two so that the motion of the apparent source may closely conform to a fiat field or to a field only slightly curved. Serious aberrations are avoided by the auxiliary stack of stationary plates. The quarter-wave matching strip between the prism and surrounding stack of plates makes possible the high effective index of refraction without serious reflections at the interface surfaces. Various known means and expedients may be employed to maintain the stacked plates in registry and stiff, so as not to vibrate or depart from registry as the rotor turns. Thus the invention teaches the use of a superior scanning structure for use with microwave energy.

What is claimed is:

1. In a scanning system for radiant electromagnetic energy comprising an antenna, a focusing device, said antenna and focusing device being arranged for the passage of energy therebetween, a rotor stack of spaced metallic plates parallel to a component of the polarization of said energy, said plates having a common axis of rotation normal to the plates and having openings therein with aligned polygonal oppositely straight edges and being positioned for the passage of said energy therethrough in a direction parallel to said plates between said focusing device and said antenna, the improvement comprising a block of solid dielectric material positioned within said aligned openings and having plane surfaces substantially parallel to said straight edges.

2. The improvement claimed in claim 1, further comprising a stator stack of metallic plates, said rotor stack of plates having circular convex outer edges centered on the center of rotation of said rotor plates, said auxiliary stack of plates having circular concave edges also centered on said center of rotation and complementary to said rotor convex circular edges, said stator plates being closely spaced in registry adjacent to said rotor plates.

3. In a scanning system for radiant electromagnetic energy comprising an antenna, a focusing device, said antenna and focusing device being arranged for the passage of energy therebetween, and a rotor stack of spaced metallic plates parallel to a component of the polarization of said energy, said plates having a common axis of rotation normal to the plates and having openings therein with aligned polygonal oppositely parallel straight edges and being positioned for the passage of said energy therethrough in a direction parallel to said plates between said focusing device and said antenna, the improvement comprising a block of solid dielectric material positioned within said aligned openings and having plane surfaces substantially parallel to said straight edges, and a different dielectric between said dielectric block and said stack of metallic plates, said different dielectric serving as a quarter-wave matching section between said dielectric block and said metallic plates.

4. The improvement claimed in claim 3, said openings being rectangular.

5. The improvement claimed in claim 3, the openings of said plates being square,

6. The improvement claimed in claim 3, said stack of metallic plates having an effective index of refraction referred to air of n,, said dielectric block having an index of refraction referred to air of n said other dielectric having an index of refraction referred to air of n,,,, n, being equal to the geometric mean between the indexes n and n 7. The improvement claimed in claim 6, said other dielectric being air and n, being equal to unity.

8. The combination of a lens system having an optical axis and a field of focus, an antenna for the radiation or reception of energy, a dielectric block between said lens and said antenna having opposed parallel plane surfaces and rotatable about an axis normal to said optical axis and parallel to said surfaces, the index of refraction of said block with respect to the surrounding medium having a value to bring the apparent position of said antenna as a source viewed from said lens system substantially into coincidence with said field, as

said block rotates.

9. The combination of a lens having an optical axis and a field of focus, an antenna for the radiation or reception of energy, a dielectric block between said lens and said antenna having opposed parallel plane surfaces and rotatable about an axis of rotation normal to said axis and parallel to said surfaces, whereby said antenna as a source viewed from said lens through said block appears as an apparent source moving with the rotation of said block, said block and said antenna being immersed in a refractive medium with respect to which the index of refraction n of said block is related by the equation:

a being an angle substantially equal to the largest angular displacement of one of said plane surfaces of said block from the position of being normal to said optical axis at which energy from or to the antenna is to be refracted through the block.

10. The improvement claimed in claim 1, further comprising a stator stack of metallic plates, said rotor stack of plates having circular convex outer edges centered on the center of rotation of said rotor plates, said auxiliary stack of plates having circular concave edges also centered on said center of rotation and complementary to said rotor convex circular edges, said stator plates being closely spaced in registry adjacent to said rotor plates, and having straight edges on the side adjacent said focusing device. 

1. In a scanning system for radiant electromagnetic energy comprising an antenna, a focusing device, said antenna and focusing device being arranged for the passage of energy therebetween, a rotor stack of spaced metallic plates parallel to a component of the polarization of said energy, said plates having a common axis of rotation normal to the plates and having openings therein with aligned polygonal oppositely straight edges and being positioned for the passage of said energy therethrough in a direction parallel to said plates between said focusing device and said antenna, the improvement comprising a block of solid dielectric material positioned within said aligned openings and having plane surfaces substantially parallel to said straight edges.
 2. The improvement claimed in claim 1, further comprising a stator stack of metallic plates, said rotor stack of plates having circular convex outer edges centered on the center of rotation of said rotor plates, said auxiliary stack of plates having circular concave edges also centered on said center of rotation and complementary to said rotor convex circular edges, said stator plates being closely spaced in registry adjacent to said rotor plates.
 3. In a scanning system for radiant electromagnetic energy comprising an antenna, a focusing device, said antenna and focusing device being arranged for the passage of energy therebetween, and a rotor stack of spaced metallic plates parallel to a component of the polarization of said energy, said plates having a common axis of rotation normal to the plates and having openings therein with aligned polygonal oppositely parallel straight edges and being positioned for the passage of said energy therethrough in a direction parallel to said plates between said focusing device and said antenna, the improvement comprising a block of solid dielectric material positioned within said aligned openings and having plane surfaces substantially parallel to said straight edges, and a different dielectric between said dielectric block and said stack of metallic plates, said different dielectric serving as a quarter-wave matching section between said dielectric block and said metallic plates.
 4. The improvement claimed in claim 3, said openings being rectangular.
 5. The improvement claimed in claim 3, the openings of said plates being square.
 6. The improvement claimed in claim 3, said stack of metallic plates having an effective index of refraction referred to air of n1, said dielectrIc block having an index of refraction referred to air of n2, said other dielectric having an index of refraction referred to air of nm, nm being equal to the geometric mean between the indexes n1 and n2.
 7. The improvement claimed in claim 6, said other dielectric being air and nm being equal to unity.
 8. The combination of a lens system having an optical axis and a field of focus, an antenna for the radiation or reception of energy, a dielectric block between said lens and said antenna having opposed parallel plane surfaces and rotatable about an axis normal to said optical axis and parallel to said surfaces, the index of refraction of said block with respect to the surrounding medium having a value to bring the apparent position of said antenna as a source viewed from said lens system substantially into coincidence with said field, as said block rotates.
 8. The combination of a lens system having an optical axis and a field of focus, an antenna for the radiation or reception of energy, a dielectric block between said lens and said antenna having opposed parallel plane surfaces and rotatable about an axis normal to said optical axis and parallel to said surfaces, the index of refraction of said block with respect to the surrounding medium having a value to bring the apparent position of said antenna as a source viewed from said lens system substantially into coincidence with said field, as said block rotates.
 9. The combination of a lens having an optical axis and a field of focus, an antenna for the radiation or reception of energy, a dielectric block between said lens and said antenna having opposed parallel plane surfaces and rotatable about an axis of rotation normal to said axis and parallel to said surfaces, whereby said antenna as a source viewed from said lens through said block appears as an apparent source moving with the rotation of said block, said block and said antenna being immersed in a refractive medium with respect to which the index of refraction n of said block is related by the equation: 